foreign-keys

I'm thinking of designing a database schema similar to the following: Person ( PersonID int primary key, PrimaryAddressID int not null, ... ) Address ( AddressID int primary key, PersonID int not null, ... ) Person.PrimaryAddressID and Address.PersonID would be foreign keys to the corresponding tables. The obvious problem is that it's impossible to insert anything into either table. Is there any way to design a working schema that enforces every Person having a primary address? "I believe this is impossible. You cannot create an Address Record until you know the ID of the person and you cannot

I am trying to create a table with a varchar column as foreign key but MySql gives me an error while creating the table. My query is like this: CREATE TABLE network_classes ( id TINYINT(1) UNSIGNED NOT NULL AUTO_INCREMENT, category VARCHAR(80) NOT NULL, PRIMARY KEY(id), KEY `key_1` (`id`,`category`) ) ENGINE=InnoDB; CREATE TABLE networks ( id TINYINT(3) UNSIGNED NOT NULL AUTO_INCREMENT, name VARCHAR(100) NOT NULL, category VARCHAR(80) NOT NULL, director_id TINYINT(3) UNSIGNED NULL, director_name VARCHAR(100) NULL, description VARCHAR(1000) NULL, last_modified TIMESTAMP NULL DEFAULT CURRENT

class First { [Key] public int Id { get; set; } } class Second { [Key] public int Id { get; set; } public int? First_Id { get; set; } [ForeignKey("First_Id")] public First First { get; set; } } public class SecondMapping : EntityTypeConfiguration<Second> { public SecondMapping () : base() { this.HasOptional(s => s.First) .With ... ??? } } Second may have a reference to First. But First never has a reference to Second. Is it possible to apply this mapping with Entity Framework 4.1? EDIT: Previously, that was my solution: this.HasOptional(s => s.First) .WithOptionalDependent()

I'm a bit of a newbie and I can't get my head around primary keys as foreign keys. To me, foreign keys are meant to connect two rows of a table together. Therefore, it would make logical sense to use the, for example, username of the user table as a foreign key in the picture table. This means that the picture in that row belongs to the specified user. However, it appears that general practice favors using meaningless numbers as primary IDs. Furthermore the foreign key must/should refer to the primary key. What if I don't know the primary key, but I know another unique column, in this case

I've been reading up on foreign keys and joins recently, and have been pleasantly surprised that many of the basic concepts are things I'm already putting into practice. For example, with one project I'm currently working on, I'm organizing word lists, and have a table for the sets, like so: `words` Table `word_id` `headword` `category_id` `categories` Table `category_id` `category_name` Now, generally speaking this would be a one-to-many relationship, with several words being placed under a single category with the foreign key category_id . Let's assume for a moment, however, that a user